Conventionally, as display elements included in a display device, there are an electro-optical element whose luminance is controlled by a voltage applied thereto, and an electro-optical element whose luminance is controlled by a current flowing therethrough. A representative example of the electro-optical element whose luminance is controlled by a voltage applied thereto includes a liquid crystal display element. On the other hand, a representative example of the electro-optical element whose luminance is controlled by a current flowing therethrough includes an organic EL (Electro Luminescence) element. The organic EL element is also called an OLED (Organic Light-Emitting Diode). An organic EL display device using organic EL elements which are self light-emitting type electro-optical elements can easily achieve slimming down, a reduction in power consumption, an increase in luminance, etc., compared to a liquid crystal display device that requires a backlight, color filters, and the like. Therefore, in recent years, there has been active development of organic EL display devices.
As the driving system of an organic EL display device, there are known a passive matrix system (also called a simple matrix system) and an active matrix system. An organic EL display device adopting the passive matrix system is simple in structure, but is difficult to achieve size increase and definition improvement. On the other hand, an organic EL display device adopting the active matrix system (hereinafter, referred to as “active matrix-type organic EL display device”) can easily achieve size increase and definition improvement, compared to the organic EL display device adopting the passive matrix system.
The active matrix-type organic EL display device has a plurality of pixel circuits formed in a matrix form. Each pixel circuit of the active matrix-type organic EL display device typically includes an input transistor that selects a pixel, and a drive transistor that controls the supply of a current to an organic EL element. Note that in the following the current flowing through the organic EL element from the drive transistor may be referred to as “drive current”.
FIG. 37 is a circuit diagram showing a configuration of a conventional general pixel circuit 81. This pixel circuit 81 is provided corresponding to each of intersections of a plurality of data lines DL and a plurality of scanning lines SL which are disposed in a display portion. As shown in FIG. 37, this pixel circuit 81 is provided with two transistors T1 and T2, one capacitor Cst, and one organic EL element OLED. The transistor T1 is an input transistor, and the transistor T2 is a drive transistor.
The transistor T1 is provided between the data line DL and a gate terminal of the transistor T2. As for the transistor T1, a gate terminal is connected to the scanning line SL, and a source terminal is connected to the data line DL. The transistor T2 is provided in series with the organic EL element OLED. As for the transistor T2, a drain terminal is connected to a power supply line that supplies a high-level power supply voltage ELVDD, and a source terminal is connected to an anode terminal of the organic EL element OLED. It should be noted that, the power supply line that supplies the high-level power supply voltage ELVDD is referred to as a “high-level power supply line” in the following, and the high-level power supply line is added with the same symbol ELVDD as that of the high-level power supply voltage. As for the capacitor Cst, one end is connected to the gate terminal of the transistor T2, and the other end is connected to the source terminal of the transistor T2. It should be noted that the other end of the capacitor Cst may be connected to the drain terminal of the transistor T2. A cathode terminal of the organic EL element OLED is connected to a power supply line that supplies a low-level power supply voltage ELVSS. It should be noted that, the power supply line that supplies the low-level power supply voltage ELVSS is referred to as a “low-level power supply line” in the following, and the low-level power supply line is added with the same symbol ELVSS as that of the low-level power supply voltage. Further, here, a contact point of the gate terminal of the transistor T2, the one end of the capacitor Cst, and the drain terminal of the transistor T1 is referred to as a “gate node VG” for the sake of convenience. It is to be noted that, although one having a higher potential between a drain and a source is generally called a drain, in descriptions of the present specification, one is defined as a drain and the other is defined as a source, and hence a source potential may become higher than a drain potential.
FIG. 38 is a timing chart for explaining an operation of the pixel circuit 81 shown in FIG. 37. Before time t81, the scanning line SL is in a non-selected state. Therefore, before the time t81, the transistor T1 is in an off state, and a potential of the gate node VG is held at an initialization level (e.g., a level in accordance with writing in the last frame). At the time t81, the scanning line SL goes into a selected state and the transistor T1 is turned on. Thereby, a data voltage Vdata corresponding to a luminance of a pixel (sub-pixel) formed by this pixel circuit 81 is supplied to the gate node VG via the data line DL and the transistor T1. Thereafter, in a period till time t82, the potential of the gate node VG changes in accordance with the data voltage Vdata. At this time, the capacitor Cst is charged with a gate-source voltage Vgs which is a difference between the potential of the gate node VG and a source potential of the transistor T2. At the time t82, the scanning line SL goes into the non-selected state. Thereby, the transistor T1 is turned off and the gate-source voltage Vgs held by the capacitor Cst is determined. The transistor T2 supplies a drive current to the organic EL element OLED in accordance with the gate-source voltage Vgs held by the capacitor Cst. As a result, the organic EL element OLED emits light with a luminance in accordance with the drive current.
Meanwhile, the organic EL display device typically adopts a thin film transistor (TFT) as a drive transistor. However, the thin film transistor is likely to have variations in characteristics (threshold voltage and mobility). When variations occur in characteristics of the drive transistors provided in the display unit, variations occur in the magnitudes of drive currents. As a result, luminance nonuniformity occurs on a display screen. Hence, in order to suppress the occurrence of luminance nonuniformity on the display screen in the organic EL display device, there is a need to compensate for variations in the characteristics of the drive transistors.
In view of this, regarding the organic EL display device, there are conventionally proposed techniques for compensating for variations in the characteristics of the drive transistors. For example, Japanese Patent Application Laid-Open No. 2007-233326 discloses an external compensation technique that enables image display with a uniform luminance regardless of the characteristics (threshold voltage and mobility) of drive transistors. In the technique disclosed in Japanese Patent Application Laid-Open No. 2007-233326, a drive current is read and control according to a result of comparison between the drive current and a data current is performed.